The invention concerns a pressing device for joining workpieces, having a pressing tool and a motorized drive for actuation of the pressing tool over a pressing distance, and having a control device which has a drive control device with an output control device as actuator for influencing the drive, such that at least one setpoint profile is retained as a command variable by means of which a manipulated variable, corresponding to the setpoint profile, is generated for influencing the output control device.
It is known, in order to join pipes, to use sleeve-like press fittings which, in order to produce a pipe joint, are slid over the pipe ends and then radially compressed, both the press fitting and the pipe being plastically deformed. Pipe joints of this kind and the pertinent press fittings are known, for example, from DE-C-11 87 870, EP-B-0 361 630, and EP-A-0 582 543.
Pressing takes place with the aid of pressing devices such as are known in various embodiments, for example from DE-C-21 36 782, DE-A-34 23 283, EP-A-0 451 806, EP-B-0 361 630, and DE-U-296 04 276.5. The pressing devices have a pressing jaw unit having at least two or sometimes more pressing jaws, which during the pressing operation are moved radially inward to form a substantially closed pressing space. The pressing tool is attached replaceably to the other part of the pressing device so that a pressing tool matching the diameter of the press fitting can be used in each case.
An electric drive, which additionally can also be combined with a hydraulic unit, is provided for movement of the pressing jaws. In the context of a pressing operation, the drive travels over a pressing distance which usually initially begins with a takeup distance before the pressing jaws come into contact against the press fitting. Over the rest of the in pressing distance, the press fitting and pipe end are deformed until a final pressed position is reached. Here the drive is automatically shut down, either by means of a force limiting element, for example in the form of a torque coupling, or a hydraulic switching valve, or by means of a limit switch in combination with a jaw closure sensor on the pressing tool (DE-U-296 02 240.3).
Known pressing devices usually have a drive control device with which the drive can only be switched on and off, i.e. in which an output control device, for the purpose of modifying the specified output over the pressing distance, is not present. In pressing devices of this kind, the pressing tool is acted upon from the outset with the fill, i.e. uncontrolled, output. The result is that a considerable quantity of kinetic energy builds up, especially in the initial phase which travels over the idle stroke. This leads to high stresses on the pressing tool, in particular in the region of the final pressed position, since there a considerable amount of kinetic energy still needs to be converted into heat. To eliminate this disadvantage, DE-U-297 03 052.3, referring to previously unpublished German Patent Application 196 33 199.4, discloses a drive control device in which, by means of an output control device, the output of the drive over the pressing distance is limited in such a way that toward the completion of pressing, the pressing tool has less kinetic energy than without output control. The result of this feature is that the maximum force acting on the parts of the pressing device moved by the drive is considerably decreased, and ideally is identical to the force to be applied during deformation of the workpieces.
In a simple embodiment, output is controlled in two stages, such that in the first phase of the pressing distance, and in particular when the takeup distance is being traveled, a low output is specified, which is then increased when the press fitting is acted upon, in accordance with the pressing resistance which thereby occurs. By storing a plurality of control specifications, a setpoint profile can be matched very closely to the profile of the pressing resistance, in such a way that the stress on the force-affected parts of the pressing device, for example when the pressing jaws encounter the press fitting, and in particular at the end of the pressing distance, are minimized (DE-U-297 03 052.3). In this context, it is also possible to store a plurality of setpoint profiles so as to be able to select the appropriate setpoint profile for every type and size of pressing tool. According to DE-U-297 03 052.3, the selection can occur manually by means of a corresponding switch, or by means of a code located on the pressing tool.
With the sequence control system described above, the output of the drive can already be matched closely to the profile of the pressing resistance, and as a result the stress on the tool can be greatly reduced and ultimately its service life can be substantially increased. It is nevertheless still necessary to provide a setpoint profile that makes excess output available under ideal conditions. The purpose of this excess output is to ensure proper pressing if conditions are less than ideal, for example if the friction conditions between pressing jaws and press fitting are unfavorable because of the material used or due to rusting, if moving parts of the pressing unit no longer run smoothly due to wear, or if the electrical voltage available is lower than usual or if voltage fluctuations occur. Under ideal conditions, the excess output makes itself apparent at a loading which is substantially lower than in the case of a drive without output control, but is still unnecessarily high.